1. Field of the Invention
The invention relates to an apparatus and a method of manufacturing bell and spigot pipe moldings from a ceramic molding compound by isostatic pressing, in which within a hollow-cylindrical pressure pot a molding cavity for the molding compound to be pressed is provided between an inner core extractable axially and an elastomeric diaphragm arranged concentrically to the latter, the diaphragm being clamped in place at the face end and defining with the pressure pot a pressure chamber exposed to a source of hydraulic pressure, the inner core being provided with a molding ring forming the bell region of the bell and spigot pipe, the molding ring comprising an end region reduced in diameter for forming two face end surface areas which are concentric but spaced away from each other axially.
2. Description of the Related Art
An apparatus of the foregoing kind is known from U.S. Pat. No. 3,193,900. That apparatus described therein is used in the isostatic pressing of bell and spigot pipe moldings. The prior art apparatus comprises a hollow-cylindrical tubular body, a molding cavity, a pressurizable elastomeric diaphragm and at its face end a steel insert for forming the bell region. In that apparatus, the bell region of the molding is produced by an annular steel insert. In the bell region, more particularly at the shoulder of the bell region, micro-cracks may occur during isostatic pressing due to the large differences in wall thickness.
There has become known from German patent DE 28 25 611 C2 (K. Strobel) an apparatus for isostatic pressing tongue and groove pipes. That apparatus comprises an inner core with a ring at the bottom end and a pressurized diaphragm. Annular inserts of a rubber or plastics material are provided for forming the tongue and groove of the pipe moldings. The purpose of the annular inserts is to transmit the pressure exerted by the elastomeric diaphragm into a wedge of the molding cavity which is not directly exposed to the elastomeric diaphragm. The result is that the molding compound located in the recess neighboring the inner core for forming a tongue is sufficiently compacted by the pressing process.
There has become known from German patent publication DE 44 07 299 C1 (K. Strobel et al.) a further apparatus for isostatically pressing tongue and groove pipes. Steel rings are provided in that apparatus as the shaping elements.